Many radio frequency (RF) signal chain circuits such as amplifiers and filters use differential signals because they are able to effectively cancel noise. A differential signal represents an instantaneous value of a signal with a positive voltage component and a corresponding negative voltage component. The difference between the positive voltage component and the negative voltage component represents the instantaneous value of the signal. The average between the positive component and the negative component is known as the common-mode voltage. While the common-mode voltage is typically ignored by differential circuits, it is important that this voltage be set to an appropriate level. For example, amplifiers and active filters work better when the common-mode voltage falls in the middle of the supply voltage so that they can have a large signal swing. However other circuits that follow the amplifiers and active filters may require higher common-mode voltages, which introduces incompatibilities. In order to shift the common-mode voltage from one circuit to another, known circuits dissipate additional current, increase noise, and make the design of the amplifier more difficult.
An example of a signal chain processing circuit that uses differential signaling is an analog-to-digital converter (ADC). ADCs convert an analog signal into a sequence of digital codes that correspond to the analog signal. In general it is important for ADCs to have a high input range. By operating with high input range, the signal-to-noise ratio of signals in the signal chain circuit can be improved.
There are several know ADC architectures, including successive approximation, pipelined, sigma-delta, resistive ladder, and ring oscillator. In ring oscillator based ADCs, the frequency of the ring oscillator (fVCO) is proportional to VIN. However fVCO is limited according to the following inequality:K*fs<fVCO<(K+1)*fs  [1]in which fs is the sampling frequency and K is a positive real number. If VIN is too low, then the ring oscillator generates just one count and if VIN is too high, then the ring oscillator wraps around. As a result, the full scale input range is limited. It would be desirable to increase the range of the ring oscillator without adding significant circuit area.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.